Process for manufacturing high strength steel

ABSTRACT

A method of making high strength steel sheet with a tensile strength of 800 to 1000 MPa and a hole expansion ratio of at least 50%, comprising the steps of reheating a previously cast slab, or retaining the heat from a directly cast slab, above Ar3; hot rolling the slab to final desired thickness; cooling the steel sheet at a rate of 50° C. per second to a temperature less than 400° C.; and winding the steel sheet into a coil.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication No. 63/283,090, filed Nov. 24, 2021, which is incorporatedherein by reference.

BACKGROUND

Typically, a direct quenching approach is employed in plate products andhot strip mill products. Tempering is applied in direct quenching of theplate and aging, for example, precipitation strengthening, have beenemployed after direct quenching in laboratory settings when developingmodels of precipitation hardening kinetics. Most of the steel producedon hot-strip mills is produced using coiling temperatures exceeding 500°C. This condition restricts the strength attainable in low alloy steels,demands higher alloy content to achieve higher strength levels ofinterest, or requires additional processing and cost through off-lineheat treatment.

In some processes, the process of manufacturing high strength steelswith good local formability, for example, bending and hole expansion inthe 800 MPa and 1000 MPa tensile strength class is produced without theneed for cold rolling.

The present disclosure includes a method of producing high strengthsteel directly on a hot-strip mill without further thermomechanicalprocessing, for example, cold-rolling and annealing. In someembodiments, the process disclosed includes utilizing low coilingtemperature, or “direct quenching,” in a hot strip mill to manufacturehigh strength steels. In some embodiments, the process described hereinincludes direct quenching, with reduced or eliminated subsequent thermaltreatment, to achieve high strength steels having fine and toughmicrostructures, for example, acicular ferrite suitable for applicationsrequiring high local formability. In some embodiments, high strengthsteel is produced, for example, bainite or martensite, directly afterquenching. In some embodiments, the strength, ductility, or toughnessbalance may be modified by subsequent tempering operations, for example,through batch annealing, continuous annealing, or hot-dip coating lines.In some embodiments, steel having fine microstructures is produced whilepreserving precipitation strengthening elements in the dissolved statefor subsequent aging treatment, similar to tempering, for example,through batch annealing, continuous annealing, or hot-dip coating lines.In some embodiments, an aging treatment will be utilized to produce adesired balance of strength, ductility, or toughness.

SUMMARY

A method of making high strength steel sheet with a tensile strength of800 to 1100 MPa and a hole expansion ratio of at least 50%, comprisingthe steps of reheating a previously cast slab, or retaining the heatfrom a directly cast slab, above Ar3; hot rolling the slab to finaldesired thickness; cooling the steel sheet at a rate of 50° C. persecond to a temperature less than 400° C.; and winding the steel sheetinto a coil.

A method of making high strength steel sheet having a tensile strengthof approximately 800 MPa and a composition of 0.06 weight percent ofCarbon, 1.0 weight percent of Mn and 0.1 weight percent of Si, 0.03weight percent of Ti and 0.0020 weight percent of Boron, comprising thesteps of: reheating a previously cast slab, or retaining the heat from adirectly cast slab, above Ar3; hot rolling the slab to final desiredthickness; cooling the steel sheet at a rate of 50° C. per second to atemperature less than 400° C.; and winding the steel sheet into a coil.

A method of making high strength steel sheet a tensile strength ofapproximately 1000 MPa and a composition 0.06 weight percent of C, 1.0weight percent of Mn, 0.1 weight percent of Si, 0.03 weight percent ofTi and 0.0020 weight percent of B, comprising the steps of: reheating apreviously cast slab, or retaining the heat from a directly cast slab,above Ar3; hot rolling the slab to final desired thickness; cooling thesteel sheet at a rate of 50° C. per second to a temperature less than400° C.; and winding the steel sheet into a coil.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a graph showing modeling prior methods of cooling atvarious positions in a coil.

FIG. 2 provides a graph showing hole expansion as a function of tensilestrength for the experimental steels without subsequent annealing.

FIG. 3 provides a graph showing hole expansion as a function of tensilestrength for the experimental steels after applying different annealingcycles.

FIG. 4 provides graphs showing the aging response via hardness testingto determine if there was a match to P* modeling, batch annealingparadigm.

FIG. 5 provides graphs showing the aging treatments conducted todetermine sensitivity to annealing.

FIG. 6 provides a graph of low temperature aging treatment conducted totemper the microstructure with the goal of improving hole expansion.

FIG. 7 provides a graph showing the batch annealing simulations todetermine sensitivity to annealing temperature.

FIG. 8 shows graphs of aging response based on batch annealing.

FIG. 9 shows a graph the aging study results conducted batch annealingsimulations with hot spot and cold spot.

FIG. 10 shows graphs of annealing screening to see sensitivity to batchannealing temperatures.

FIG. 11 provides a graph of the annealing simulation with hot spot andcold spot cycle.

FIG. 12 shows graphs of lower anneal temperatures.

DETAILED DESCRIPTION

In some embodiments, the process of manufacturing high strength steelsrequires developing a ferritic microstructure that is substantiallystrengthened by precipitation hardening. The principal precipitationhardening prior processes are either titanium based, or vanadium based.These technologies employ common hot-strip mill (HSM) processing, withcoiling temperatures of at least 600° C. (1112° F.). Free cooling of ahot coil, as is the conventional practice, inherently results in varyingtime-temperature history for the different positions in the coil. Theextremities of the coil (edges, outer wraps in particular) cool morerapidly than the coil interior. Such variations can be estimated in theprior methods, as shown in FIG. 1 . This example is based on initialcoiling temperature of 1325° F., 30-inch coil inner diameter, 65.6-inchouter diameter, and 0.371-inch thickness. This has significantimplications for the precipitation hardening reactions upon which thismaterial design relies and induces undesirable mechanical propertyvariability.

In some prior methods, the addition of molybdenum may mitigate thevariability for titanium carbide precipitates and in some examplesvanadium-based precipitates. It is known that acicular ferritemicrostructures offer combinations of strength and toughness. Thesemicrostructures are the underpinning of line pipe products. Localformability is effectively a measure of toughness and high strengthsteels as disclosed herein.

Acicular ferrite can be developed by quenching low carbon steels, andquenching strip to a low temperature before winding in the coilermitigates the variability in post-coiling cooling. The presentdisclosure discusses direct quenching steel after hot strip millprocessing. In some embodiments, direct quenching may preserveprecipitation hardening species in a dissolved state (unprecipitated).

The primary purpose of a hot strip mill is to reheat thick steel slabsinto thin sheets with varying thickness. The thick steel slab passesthrough several rolling mill stands that are driven by powerful motors.The rolled sheets then pass through coilers, thereafter these coils moveon to the next process in the plant. From the startup to the end, thesteel material undergoes several treatments through each stage that arethe main features of a hot strip mill.

The disclosure includes a method of inducing precipitation strengtheningreactions under controlled thermal conditions, such as batch annealing,continuous annealing, or adjusting properties with improved uniformity.Additionally, the disclosure includes data showing that annealingquench-and-tempered products are shown to achieve a combination ofstrength and toughness.

The disclosure is applicable to a broad range of steel hot rollingprocesses. In some embodiments, the steel is hot rolled while the steelis primarily in its austenitic state and that the rolled strip issubsequently cooled to a temperature low enough, and at a sufficientrate, to achieve acicular ferrite or bainitic structures. In someembodiments, a precursor for final hot rolling can be produced in tandemwith final rolling sequence (direct casting and rolling technologieswith or without intermediate reheating in advance of the final rolling)or can be produced in an independent facility with the slabs or transferbars reheated for processing in a hot strip mill.

In some embodiments, the temperature of the final rolling step should besuch that the steel is in the austenitic start. This causes the lastrolling pass to be completed at a temperature greater than theaustenite-to-ferrite transformation temperature, also known astemperature Ar3.

In some embodiments, upon completion of the final rolling step, thesteel is rapidly cooled to achieve the desired acicular ferrite and/orbainite microstructure (depending on strength class). The rapid coolingcontinues until the steel is less than 400° C. The steel strip is thenwound into a coil. The rate of rapid cooling should be greater than 50°C/second.

In some embodiments, a secondary treatment process is applied to thesteel strip to promote precipitation reactions for strength preservationor increase. In this embodiment, the hot rolled strip should be reheatedto a temperature above 500° C. and below the ferrite-to-austenite phasetransformation temperature, for example, a temperature Ac1. Theappropriate temperature depends on the time duration anticipated for theprocess employed. For example, continuous annealing of the steel stripwill result in shorter heating times than batch annealing of coils. Theshorter duration of continuous annealing operations (for hot-dip coatedor uncoated strip) allows the strip to approach the Ac1 temperaturewhile achieving the desired properties.

In some embodiments, the steel composition includes carbon. In someembodiments, the steel composition includes carbon in a range ofapproximately 0.03 to 0.07 weight percent. Carbon levels belowapproximately 0.03 weight percent will risk the ability to achieve thedesired strength level. Higher levels of carbon risk low hole expansionperformance and can make the steel prone to the adverse peritecticreaction during continuous casting.

In some embodiments, the steel composition includes manganese. In someembodiments, the steel composition includes manganese in a range ofapproximately at most 2.0 weight percent. Manganese is one of the moreeconomical strengthening elements that also sequesters sulfur preventthe formation of damaging iron sulfide. A minimum practical level forhigher strength steels is approximately 0.5 weight percent, andeconomics often dictate higher levels to preclude the use of more costlyelements. Elevated levels of manganese lead to chemical segregationpatterns that can be damaging to performance.

In some embodiments, the steel composition includes molybdenum. In someembodiments, the steel composition includes molybdenum in a range of atmost approximately 0.5 weight percent. Molybdenum is a potentstrengthening element, but often expensive to employ. It may be chosento limit the maximum manganese content employed or to add thermalstability to precipitation hardening species. If not technicallyrequired, a residual level would be employed for economic reasons.

In some embodiments, the steel composition includes chromium. In someembodiments, the steel composition includes chromium less thanapproximately 2.0 weight percent. Chromium is a potent strengtheningelement. Economics often suggest its use after manganese but beforemolybdenum. It can be used to limit the maximum manganese employed. Forthis technology, additions less than approximately 2.0 weight percentare appropriate.

In some embodiments, the steel composition includes silicon. In someembodiments, the steel composition includes silicon less than up toapproximately 1 weight percent silicon is an efficient strengtheningelement. Higher levels of silicon can induce surface features on the hotstrip that may be objectionable depending on the application. Highersilicon levels can also interfere with galvanizing operations.

In some embodiments, the steel composition includes boron. In someembodiments, the steel composition includes boron less than up toapproximately in the range of 10 to 30 parts per million. Thestrengthening effect can only be assured with use of a nitrogensequestering element, most typically titanium. The sequestering ofnitrogen results in coarse nitride particles that can be damaging to thetoughness of the steel. As such, the use of boron alloying may not beappropriate for the most toughness critical applications.

In some embodiments, the steel composition includes titanium. In someembodiments, the steel composition includes titanium as a potentstrengthening element. In the context of this disclosure, titanium isprincipally utilized as a nitrogen sequestering element to facilitatethe use of boron, or as a precipitation strengthener for secondarythermal operations. The appropriate level for use in nitrogensequestration is at a level of 3.4 time the nitrogen content of thesteel. A practical maximum addition for the precipitation strengtheningconsideration would be 0.2 weight percent.

In some embodiments, the steel composition includes vanadium. In someembodiments, the steel composition includes vanadium at approximately0.2 weight percent. Vanadium can be a potent strengthening element. Inthe context of this disclosure, the use of vanadium is as aprecipitation strengthener for secondary thermal operations.

In some embodiments, the steel composition includes copper. In someembodiments, the steel composition includes copper at approximately inthe range of 0.3 to 0.5 weight percent where atmospheric corrosionresistance is desired. Copper is not considered a critical strengtheningelement. In the context of the disclosure, copper would only be employedwhen atmospheric weathering resistance is desired. Suitable mechanicalproperties can be achieved without the need for this costly alloyingelement. The use of copper must be judicious as it can result in lowductility during hot rolling operations (hot shortness). Depending onthe hot rolling process employed, a concurrent nickel addition may bemandatory to mitigate the hot ductility reduction.

In some embodiments, the steel composition includes nickel. In someembodiments, the steel composition includes nickel at approximately thelevel of one-half the copper addition. This level has been foundsuitable for mitigating the low ductility at hot rolling temperatures.Nickel additions can be employed for strengthening, toughening, or tomitigate low ductility during hot rolling. In the context of thedisclosure, nickel would only be employed as a companion to copperadditions when atmospheric weathering resistance is desired. Suitablemechanical properties can be achieved without the need for this costlyalloying element.

In some embodiments, the steel composition includes a tensile strengthof approximately 800 MPa, a very economical steel composition would beapproximately (all values in weight percent):0.06C-1.0Mn-0.1Si-0.03Ti-0.0020B; with no additional intentionaladditions. Using a similar alloy design for nominally 1000 MPa tensilestrength, the composition would be approximately (all values in weightpercent): 0.06C-1.0Mn-0.1Si-0.03Ti-0.0020B; with no additionalintentional additions. Alternative designs without boron additions canbe considered. For example, 800 MPa tensile strength steel would beexpected with a composition of approximately (all values in weightpercent): 0.06C-1.5Mn-0.1Si, with no additional intentional additions.To reach the 1000 MPa tensile strength level the manganese level wouldbe increased to its practical maximum of 2.0 weight percent and chromiumwould be added at a level of 0.5 weight percent.

As described herein, direct-quenching is a first step of the heattreatment operation with subsequent tempering occurring in a differentprocess step (batch annealing, continuous annealing). This approach doesnot rely on precipitation hardening reactions and is an alternativeimplementation of known quench-and-temper concepts.

FIG. 2 illustrates the combination of two key properties of primaryinterest: hole expansion and tensile strength. FIG. 2 illustrates agraph showing hole expansion as a function of tensile strength. Thegraphs show properties of the steel manufactured according to anembodiment of the process of the present disclosure in theas-direct-quenched condition and after annealing. The present disclosureis configured to produce steel having at least 800 MPa tensile strength,with hole expansion of at least 50%. The graphs shown in FIG. 2 showproperties without subsequent annealing, the second graph showsproperties after applying different annealing cycles. The graphs in FIG.2 illustrate that there are various batches that produced good holeexpansion at tensile strength greater than 800MPa.

The following examples are intended to illustrate various aspects of thepresent disclosure and are not intended to limit the scope of thedisclosure. Many different steel alloys were considered. The strength ofthe direct-quenched product can be expected to vary as a function ofcomposition. Table 1 shows a regression model for tensile strength as afunction of composition. The data is sorted by ascending P-Value,placing the elements of most significance to the regression at the topof the list (higher P-Value means higher probability of randomcontribution).

Table 1 illustrates regression results of tensile strength vscomposition for as-direct-quenched plates.

TABLE 1 Standard Error Term Coefficient of the Coefficient T-ValueP-Value VIF Constant 322.2 80.1 4.02 0 C 3688 665 5.55 0 2.35 B 8283016566 5.00 0 2.48 Mn 130.1 38.9 3.35 0.002 1.87 Mo 256 126 2.03 0.0483.40 Cr 118.7 67.7 1.75 0.086 3.90 Cb −435 320 −1.36 0.180 2.38 Si 64.959.2 1.10 0.278 2.90 Cu 158 492 0.32 0.749 74.05 Ni −192 850 −0.23 0.82264.92 V −14 443 −0.03 0.975 1.68 Ti 1 208 0 0.997 2.27

The inclusion of C, B, Mn, Mo, and Cr are utilized to increase thehardenability of the steel. In some embodiments, the contribution of Cb,particularly as indicated by a negative coefficient, reflects thiselement's contribution to grain size refinement and a negativecontribution to hardenability. In some embodiments, Cu and Ni additions,while expected to contribute to hardenability, were not reported asreliable contributors (quite high P-Value). Similarly, in someembodiments, V and Ti had high P-Values. This condition is reasonablefor V and Ti since their contributions in these steels is primarilythrough precipitation hardening, which is not expected to be active inthe as-quenched condition. It is through subsequent aging treatmentsthat V and Ti, as well as Cb, will contribute to strength preservationor increase.

Tables 2 and 3 illustrate data from Campaign 1. Plates were hot rolledand direct quenched to room temperature. Heat compositions (all valuesin weight percent).

TABLE 2 Heat Note C Mn P S Si Cu Ni Cr Mo V Ti Al N Cb B Pcm 8774A HICSteel 0.046 1.07 0.009 0.0031 0.20 0.30 0.20 0.50 0.01 0.005 0.015 0.030.0051 0.030 0.0002 0.151 8774B Development: 0.047 1.08 0.008 0.00260.20 0.30 0.20 0.51 0.01 0.005 0.015 0.04 0.0052 0.059 0.0002 0.1538774C Based on 0.047 1.08 0.008 0.0026 0.20 0.30 0.20 0.51 0.01 0.0050.015 0.04 0.0049 0.090 0.0002 0.153 8775B lower Mn 0.043 0.80 0.0090.0030 0.19 0.02 0.01 0.24 0.01 0.005 0.014 0.03 0.0048 0.061 0.00020.105 8775C 0.046 0.80 0.009 0.0023 0.19 0.02 0.01 0.48 0.01 0.005 0.0140.03 0.0048 0.062 0.0002 0.120

TABLE 3 Uni. Total Hole Thickness YS UTS Elong. Elong. Expansion* Heat(in.) (MPa) (MPa) (%) (%) (%) 8774A 0.250 502 684 10.0 28.4 69 8774B0.250 488 685 10.1 30.7 69 8774C 0.250 541 730  9.5 28.3 52 8775B 0.250455 631 12.7 30.9 94 8775C 0.250 482 641 11.7 32.5 79 *plates wereground to 5 mm thick prior to hole expansion testing.

Tables 4 and 5 illustrate data from Campaign 2. Plate was hot rolled anddirect quenched to room temperature. Subsequently did aging treatmentsto determine sensitivity to annealing.

TABLE 4 Note C Mn P S Si Cu Ni Cr Mo V Ti Al N Cb B P_(cm) Stock slab.064 1.82 .013 .0032 .06 .01 .04 .03 .00 .020 .112 .02 .0047 .063 .0001.162 from HSMM Studies

TABLE 5 Uni. Thickness (in.) YS (MPa) UTS (MPa) Elong. (%) Total Elong.(%) 0.250 577 784 7.6 23.2 No hole expansion testing conducted.

FIG. 4 provides graphs of an aging response via hardness testing todetermine if there was a match to P* modeling. Batch annealing paradigmtimes were 1 hour through 48 hours, 3600 s to 172800 s. Hardness testsconducted using HRA scale, converted to HRC.

Tables 6 and 7 illustrate data from Campaign 3. Plates were hot rolledand direct quenched.

TABLE 6 Heat Note C Mn p S Si Cu Ni Cr Mo V Ti Al N Cb B Pcm 8709 USSC0.048  1.65  0.010 0.0050 0.28  0.30  0.15  0.30  0.20 0.02  0.020 0.034 0.0071 0.085  0.0003 0.189 8710 X70 0.044  1.68  0.010 0.00430.29  0.30  0.15  0.45  0.01 0.02  0.021  0.036 0.0061 0.084  0.00020.182 8711 Lab 0.050  1.64  0.010 0.0045 0.29  0.28  0.15  0.45  0.150.02  0.021  0.030 0.0071 0.089  0.0002 0.194 8712 Heats 0.035  1.93 0.010 0.0043 0.30  0.31  0.15  0.46  0.01 0.02  0.022  0.030 0.00540.087  0.0001 0.185 8713 0.047  1.91  0.010 0.0045 0.30  0.30  0.15 0.45  0.15 0.02  0.020  0.042 0.0070 0.085  0.0003 0.206 8714 0.048 1.88  0.010 0.0043 0.30  0.30  0.15  0.44  0.01 0.02  0.026  0.0320.0070 0.086  0.0016 0.202 9183 LTO 0.093 1.412 0.0131 0.0042 0.3940.031 0.011 0.0637 0.07 0.0552 0.0072 0.036 0.0082 0.0319 0.19  Plate BX65 9183 Lab 0.093 1.412 0.0131 0.0042 0.394 0.031 0.011 0.0637 0.070.0552 0.0072 0.036 0.0082 0.0319 0.19  Plate D Heats 9184 0.082 1.3620.0095 0.0021 0.405 0.03 0.01 0.032 0.081 0.0647 0.0006 0.028 0.00720.020 0.18  Plate B 9184 0.082 1.362 0.0095 0.0021 0.405 0.03 0.01 0.0320.081 0.0647 0.0006 0.028 0.0072 0.020 0.18  Plate C 9184 0.082 1.3620.0095 0.0021 0.405 0.03 0.01 0.032 0.081 0.0647 0.0006 0.028 0.00720.020 0.18  Plate D

TABLE 7 Uni. Total Hole Thickness YS UTS Elong. Elong. Expansion* Heat(in.) (MPa) (MPa) (%) (%) (%) 8709 0.25 697 861 6.0 21.9 54 8710 0.25613 793 6.7 23.6 67 8711 0.25 674 861 6.8 22.8 46 8712 0.25 622 798 5.514.3 79 8713 0.25 670 849 6.2 60 8714 0.25 863 1000  3.8 16.8 46 9183Plate B 0.25 577 851 10.3 23.8 25 9183 Plate D 0.178  568 859 7.9 14.334 9184 Plate B 0.25 726 882 3.6 17.7 42 9184 Plate C 0.17 524 792 9.016.2 42 9184 Plate D 0.18 540 798 9.6 20.8 43 *0.250″ thick platesground to 5 mm prior to hole expansion testing.

Tables 8 and 9 illustrate data from Campaign 4. Hot rolled to heavygauge and initial testing conducted.

TABLE 8 Heat Note C Mn P S Si Cu Ni Cr Mo V Ti Al N Cb B Pcm 8709 USSC0.048 1.65  0.010  0.0050 0.28  0.30  0.15  0.30  0.20  0.02  0.020 0.034 0.0071 0.085  0.0003 0.189 8710 X70 0.044 1.68  0.010  0.00430.29  0.30  0.15  0.45  0.01  0.02  0.021  0.036 0.0061 0.084  0.00020.182 8711 Lab 0.050 1.64  0.010  0.0045 0.29  0.28  0.15  0.45  0.15 0.02  0.021  0.030 0.0071 0.089  0.0002 0.194 8712 Heats 0.035 1.93 0.010  0.0043 0.30  0.31  0.15  0.46  0.01  0.02  0.022  0.030 0.00540.087  0.0001 0.185 8713 0.047 1.91  0.010  0.0045 0.30  0.30  0.15 0.45  0.15  0.02  0.020  0.042 0.0070 0.085  0.0003 0.206 8714 0.0481.88  0.010  0.0043 0.30  0.30  0.15  0.44  0.01  0.02  0.026  0.0320.0070 0.086  0.0016 0.202 9183 LTO 0.093 1.412 0.0131 0.0042 0.3940.031 0.011 0.0637 0.07  0.0552 0.0072 0.036 0.0082 0.0319 0.19  Plate BX65 9183 Lab 0.093 1.412 0.0131 0.0042 0.394 0.031 0.011 0.0637 0.07 0.0552 0.0072 0.036 0.0082 0.0319 0.19  Plate D Heats 9184 0.082 1.3620.0095 0.0021 0.405 0.03 0.01 0.032 0.081 0.0647 0.0006 0.028 0.00720.020 0.18  Plate B 9184 0.082 1.362 0.0095 0.0021 0.405 0.03 0.01 0.0320.081 0.0647 0.0006 0.028 0.0072 0.020 0.18  Plate C 9184 0.082 1.3620.0095 0.0021 0.405 0.03 0.01 0.032 0.081 0.0647 0.0006 0.028 0.00720.020 0.18  Plate D

TABLE 9 YS UTS Uni. Total Heat Thickness (in.) (MPa) (MPa) Elong. (%)Elong. (%) 9081a 0.268 620 863 7.6 19.4 9081b 0.265 643 879 7.0 22.29082a 0.259 633 816 6.1 20.1 9082b 0.257 809 978 3.7 14.9 9083a 0.263581 789 7.7 24.2 9083b 0.261 610 805 6.8 18.6 9084a 0.261 567 792 7.122.9

Table 10 provides data for plates that were ground to 5 mm thick tofacilitate hole expansion tests. Subsize longitudinal tensile specimensextracted from edges and tested.

TABLE 10 Uni. Hole Thickness YS UTS Elong. Total Elong. Expansion Heat(in.) (MPa) (MPa) (%) (%) (%) 9081a 0.197 715 908 5.5 19.8 46 9081b0.197 606 823 7.1 19.9 50 9082a 0.198 717 896 5.0 16.8 66 9082b 0.202698 872 5.3 17.9 67 9083a 0.200 701 880 5.4 17.2 58 9083b 0.198 647 8286.7 19.8 66 9084a 56

FIG. 5 shows graphs of an aging treatment conducted to determinesensitivity to annealing. All tests were conducted in salt pots andhardness measure by HRA. Converted to VHN to allow more directestimation of tensile strength. FIGS. 6 and 7 simulate batch annealingof hot spots and cold spots subjected to a low temperature agingtreatment conducted to temper the microstructure with the goal ofimproving hole expansion.

Table 11 illustrates hole expansion after low temperature tempertreatments.

TABLE 11 Hole Expansion (%) Heat Before tempering Hot Spot, 272° F. ColdSpot, 185° F. 9081a 46 41 40 9081b 50 48 54 9082a 66 68 69 9082b 67 7868 9083a 58 60 66 9083b 66 56 58 9084a 56 60 53

Tables 12 and 13 illustrate data from Campaign 5. Plates hot rolled anddirect quenched to room temperature.

TABLE 12 Heat Note Alloy C Mn P S Si Cu Ni Cr 8976 Existing FW11 0.0621.648 0.009 0.0033 0.181 0.017 0.099 0.2 8977 Line Pipe FM25 0.054 1.520.015 0,0037 0.233 0.016 0.139 0.202 9026 Grades XM02 0.053 1.51 0.0110.0025 0.198 0.021 0.02 0.03 9027 FM22 0.055 1.507 0.012 0.0044 0.210.02 0.019 0.19 9028 FW18 0.053 1.494 0.011 0.0043 0.197 0.099 0.2010.25 Heat Note Mo V Ti Al N Cb B Pcm 8976 Existing 0.141 0.039 0.0120.031 0.0059 0.051 0.1762 8977 Line Pipe 0.149 0.0025 0.0118 0.0330.0055 0.075 0.1612 9026 Grades 0.011 0.0021 0.0149 0.033 0.0048 0.08610.1389 9027 0.013 0.0023 0.0166 0.031 0.0044 0.0837 0.1493 9028 0.1510.04 0.016 0.03 0.005 0.05 0.1691

TABLE 13 Finishing Uni. Total Hole Temp. Thickness YS UTS Elong. Elong.Expansion Heat Note (º F.) (in.) (MPa) (MPa) (%) (%) (%) 8976 Plate A1515 0.172 573 797 8.8 22.3 57 Plate B 1605 0.178 597 813 8.2 20.7 558977 Plate A 1525 0.172 562 767 9.5 22.8 60 Plate B 1600 0.177 572 7708.8 22.9 56 9026 Plate A 1520 0.170 534 715 11.0 22.9 68 Plate B 16100.176 541 713 10.7 24.9 77 9027 Plate A 1520 0.170 563 728 10.8 26.1 67Plate B 1625 0.175 543 734 10.3 26.0 57 9028 Plate A 1525 0.169 578 7828.7 20.8 72 Plate B 1595 0.175 590 785 8.9 23.3 65

Tables 14 and 15 illustrate data from Campaign 6. Plates were hot rolledand quenched to room temperature.

TABLE 14 Heat Note C Mn P S S Cu Ni Cr Mo V Ti Al N Cb B Pcm 8709 USSC0.048 1.645 0.010 0.0050 0.282 0.296 0.146 0.299 0.196 0.0220 0.0200.034 0.0071 0.085 0.0003 0.189 8710 X70 Trials 0.044 1.677 0.010 0.00430.293 0.296 0.148 0.451 0.014 0.0220 0.021 0.036 0.0061 0.084 0.00020.182 8711 0.050 1.644 0.010 0.0045 0.292 0.278 0.147 0.453 0.146 0.02100.021 0.030 0.0071 0.089 0.0002 0.194 8712 0.035 1.930 0.010 0.00430.295 0.308 0.145 0.455 0.011 0.0230 0.022 0.030 0.0054 0.087 0.00010.185 8713 0.047 1.910 0.010 0.0045 0.300 0.299 0.150 0.446 0.152 0.02200.020 0.042 0.0070 0.085 0.0003 0.206 8714 0.048 1.884 0.010 0.00430.304 0.299 0.150 0.444 0.011 0.0200 0.026 0.032 0.0070 0.086 0.00160.202 8681A Lab HTP- 0.037 1.582 0.011 0.0056 0.195 0.030 0.024 0.2830.016 0.0020 0.015 0.028 0.0066 0.087  0.000 0.140 8681B Type Heats,0.057 1.564 0.011 0.0054 0.194 0.030 0.024 0.282 0.016 0.0020 0.0150.026 0.0065 0.085 0.0001 0.159 8682A C-Cr Study 0.032 1.614 0.0120.0041 0.202 0.031 0.008 0.501 0.016 0.0030 0.015 0.031 0.0050 0.0880.0001 0.148 8682B 0.056 1.643 0.010 0.0040 0.209 0.031 0.008 0.5030.015 0.0030 0.016 0.028 0.0046 0.090 0.0001 0.174 8683A 0.027 0.5720.011 0.0050 0.199 0.031 0.008 0.705 0.013 0.0030 0.015 0.029 0.00570.090 0.0001 0.151 8683B 0.050 1.561 0.010 0.0050 0.198 0.030 0.0080.700 0.013 0.0030 0.015 0.026 0.0057 0.088 0.0001 0.173

TABLE 15 Uni. Total Hole Thickness YS UTS Elong. Elong. Expansion Heat(in.) (MPa) (MPa) (%) (%) (%) 8709 0.173 626 819 7.2 19.3 42 8710 0.171618 799 8.1 22.1 52 8711 0.171 643 848 7.5 19.8 40 8712 0.170 611 7857.6 21.6 61 8713 0.169 661 843 7.0 19.4 52 8714* 0.170 573 758 7.4 19.549 8681A** 0.174 568 703 9.5 23.9 63 8681B 0.172 557 742 10.1 24.6 498682A 0.170 567 701 8.3 22.2 78 8682B 0.170 580 778 9.1 23.6 55 8683A0.169 555 701 8.1 20.4 71 8683B 0.167 592 784 8.3 21.5 49 *Not properlydirect quenched, speed under sprays too high. **Delay during rollingresulted in ultra-low finishing temperature.

Tables 16 and 17 illustrate a direct quench portion of 780 developmentbainitic approach. Lab heats hot rolled with two different finishingtemperatures and direct quenched to room temperature.

TABLE 16 Heat Note C Mn P S Si Cu Ni Cr Mo V Ti Al N Cb B Pcm 9304 FM370.058 1.548 0.012  0.0034 0.245 0.02  0.01  0.033 0.217 0.0016 0.0230.031 0.0067 0.09  0.002  0.171 Base 9305 Low N 0.062 1.617 0.011 0.0034 0.248 0.02  0.011 0.034 0.22 0.0016 0.024 0.033 0.0035 0.092 0.0019 0.178 FM37 9306 FM 37, 0.059 1.58 0.011  0.0036 0.241 0.019 0.01 0.033 0.218 0.0015 0.024 0.032 0.0037 0.045  0.0018 0.172 low N, Nb 9307Mesplont 0.063 1.585 0.012  0.0031 0.253 0.02  0.01  0.033 0.149 0.00150.023 0.031 0.0036 0.045  0.0019 0.173 low N 9308 Babbit 0.04  1.7790.0099 0.0037 0.247 0.019 0.011 0.034 0.294 0.0018 0.023 0.031 0.00670.06  0.002  0.170 1992 9309 Nunakawa 0.11  1.604 0.011  0.0029 0.5190.02  0.01  0.486 0.011 0.002  0.071 0.032 0.0034 0.0022 0.0004 0.2361985

TABLE 17 Uni. Total Hole Finishing Thickness YS UTS Elong. Elong.Expansion Heat Note Temp. (º F.) (in.) (MPa) (MPa) (%) (%) (%) 9304Plate A 1545 0.185 714 901 6.7 15.5 40 Plate H 1650 0.184 816 995 5.011.1 34 9305 Plate A 1555 0.182 663 878 6.7 14.8 41 Plate H 1665 0.184779 1013 5.5 12.1 37 9306 Plate A 1520 0.182 746 911 3.6 10.8 35 Plate H1635 0.181 1023 4.5 11.0 38 9307 Plate A 1555 0.179 787 945 4.6 11.4 55Plate H 1640 0.180 850 1022 5.2 12.6 35 9308 Plate A 1540 0.180 756 8984.5 11.3 44 Plate H. 1650 0.177 861 976 4.7 11.2 46 9309 Plate A 15300.178 796 1068 5.5 10.8 24 Plate H 1625 0.175 862 1162 5.1 11.8 30

FIG. 8 provides graphs showing an aging response based on batchannealing paradigm. In this embodiment, batch annealing was conducted todetermine sensitivity to the annealing temperature, for example, heat at100° F./hr., hold 24 hours, furnace cool.

FIG. 9 shows results based on the aging study results, conducted batchannealing simulations with hot spot and cold spot. In some embodiments,the hot spot temperature was 1100° F., around peak aging with Mo steels,overaging without Mo. In some embodiments, the cold spot was 1000° F.,below peak aging, if peak aging was 24 hours at 1000° F.

Table 18 shows results based on the aging study results, conducted batchannealing simulations with hot spot and cold spot, a hot spot of 1100°F. was tested at approximately peak aging with Mo steels, overagingwithout Mo. In addition, a cold spot of 1000° F. was tested. This isbelow peak aging if peak aging was 24 hours at 1000° F.

TABLE 18 Alloy Condition YS UTS TE HE 9304 DQ 714 901 15.5 40 Cold Spot803 864 35 Hot Spot 737 786 39 9305 DQ 663 878 14.8 41 Cold Spot 833 89435 Hot Spot 785 803 40 9306 DQ 746 911 10.8 35 Cold Spot 832 879 36 HotSpot 769 804 35 9307 DQ 787 945 11.4 55 Cold Spot 823 866 38 Hot Spot737 780 42 9308 DQ 756 898 11.3 44 Cold Spot 823 871 42 Hot Spot 780 81039 9309 DQ 796 1068 10.8 24 Cold Spot 899 960 32 Hot Spot 751 801 33

Tables 19 and 20 show results of direct quench portion of HR780development, hybrid of FM13 and KSL 780R. The data includes lab heatsfor CAL/HD grades. Plates were hot rolled with different finishingtemperatures and direct quenched to room temperature.

TABLE 19 Heat Note C Mn P S Si Cu Ni Cr Mo V Ti Al N Cb B Pcm 9324 FM130.053 1.819 0.012 0.0024 0.117 0.022 0.01  0.044 0.005  0.003 0.109 0.031 0.0068 0.034  0.0004 0.154 9325 FM13 + Si 0.055 1.827 0.011 0.00230.52  0.021 0.01  0.044 0.005  0.003 0.109  0.034 0.0071 0.034  0.00040.170 9326 FM13 + 0.055 1.792 0.011 0.0022 0.52  0.021 0.011 0.0420.0056 0.003 0.111  0.032 0.0065 0.053  0.0004 0.168 Si + Nb 9327 FM13 +Si + 0.056 1.788 0.012 0.002 0.513 0.021 0.01  0.042 0.0054 0.004 0.175 0.034 0.0064 0.053  0.0004 0.169 Nb + Ti 9328 KSL780R + 0.084 1.7930.012 0.0021 0.508 0.021 0.011 0.042 0.0055 0.004 0.175  0.033 0.00650.053  0.0004 0.197 Mn + Si 9329 KY06 0.085 1.855 0.012 0.0021 0.8730.021 0.01  0.042 0.0051 0.002 0.0016 0.034 0.0067 0.0044 0.0004 0.2139330 KT24 0.075 1.899 0.012 0.002  0.101 0.02  0.011 0.198 0.171  0.0020.0008 0.03  0.0067 0.0038 0.0004 0.198

TABLE 20 Uni. Total Hole Finishing Thickness YS UTS Elong. Elong.Expansion Heat Note Temp. (º F.) (in.) (MPa) (MPa) (%) (%) (%) 9324Plate C 1520 0.185 600 789 7.8 17.8 61 Plate J 1630 615 788 7.6 14.6 479325 Plate C 1505 622 832 7.9 17.4 54 Plate I 1630 639 834 6.9 15.2 429326 Plate C 1505 618 828 8.0 18.1 52 Plate H 1630 562 828 6.0 14.7 419327 Plate C 1515 639 834 8.2 17.1 52 Plate H 1650 482 835 6.5 16.2 479328 Plate C 1580 635 886 8.6 16.9 34 Plate H 1635 615 868 7.0 14.3 379329 Plate C 1480 512 866 11.5 19.9 19 Plate H 1615 591 973 7.4 15.0 299330 Plate C 1615 756 975 3.6 10.2 46 Plate H 1750 846 1037 4.3 11.5 42

FIG. 10 shows graphs of an annealing screening to determine sensitivityto batch annealing temperatures of 900, 1000, 1100, 1200° F. at 24hours. FIG. 11 shows a batch annealing simulation with hot spot and coldspot cycle. Table 21 includes data from a batch annealing simulationwith hot spot and cold spot cycle.

TABLE 21 Alloy Condition YS (MPa) UTS (MPa) TE (%) HE (%) 24 DQ 600 78917.8 61 CS 783 838 21.1 42 HS 747 794 21.0 58 25 DQ 622 832 17.4 54 CS815 876 20.9 37 HS 773 820 23.2 38 26 DQ 618 828 18.1 52 CS 819 875 20.737 HS 765 816 23.2 37 27 DQ 639 834 17.1 52 CS 852 914 21.7 40 HS 821862 22.7 48 28 DQ 635 886 16.9 34 CS 848 911 20.1 40 HS 793 846 20.2 4229 DQ 512 866 19.9 19 700CS 593 758 20.8 42 800HS 579 705 23.3 47 650CS648 840 19.5 80 750HS 639 783 22.5 65 30 DQ 756 975 10.2 46 700CS 725793 12.9 69 800HS 710 779 16.1 72 650CS 803 911 14.4 93 750HS 842 90416.4 86

FIG. 12 shows data for lower anneal temperatures in Heat 30, 900° F. for24 hours results in approximately 755 MPa. Table 22 illustrates a directquench portion of HR780 development including Ti with different Nlevels. These heats were for tuning FM13/FM44. Lab heats hot rolled anddirect quenched to room temperature.

TABLE 22 Heat Note C Mn P S Si Cu Ni Cr 9420A Low Ti, 0.0676 1.937350.01192 0.00153 0.49234 0.02033 0.01034 0.04169 Mid N 9420B Mid Ti0.0672 1.94201 0.01166 0.0015 0.49923 0.02077 0.0103 0.04158 Mid N 9420CHigh Ti, 0.0682 1.9289 0.01144 0.00154 0.49305 0.02023 0.01018 0.04111Mid N 9435A Low Ti 0.0601 1.86559 0.01151 0.0014 0.47612 0.01718 0.009980.04111 Low N 9435B Mid Ti, 0.0603 1.86451 0.01147 0.00349 0.474010.01716 0.00997 0.04111 Low N 9435C High Ti, 0.0587 1.87687 0.010970.00136 0.48274 0.01764 0.00945 0.04096 Low N Heat Note Mo V Ti Al N CbB Pcm 9420A Low Ti, 0.0055 0.00991 0.07823 0.04935 0.0066 0.032830.00028 0.187 Mid N 9420B Mid Ti 0.0056 0.01044 0.10243 0.05149 0.006430.0328 0.0003 0.187 Mid N 9420C High Ti, 0.00532 0.01051 0.12744 0.050830.0059 0.0318 0.00028 0.187 Mid N 9435A Low Ti 0.00509 0.00906 0.071260.03824 0.00425 0.03242 0.00026 0.175 Low N 9435B Mid Ti, 0.005170.00937 0.09294 0.0376 0.00449 0.03256 0.00027 0.175 Low N 9435C HighTi, 0.00484 0.00993 0.1167 0.03667 0.00443 0.03167 0.00026 0.174 Low N

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the invention are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical value, however, inherently contains certainerrors necessarily resulting from the standard variation found in theirrespective testing measurements.

Also, any numerical range recited herein is intended to include allsub-ranges subsumed therein. For example, a range of “1 to 10” isintended to include all sub-ranges between (and including) the recitedminimum value of 1 and the recited maximum value of 10, that is, havinga minimum value equal to or greater than 1 and a maximum value of equalto or less than 10.

In this application, the use of the singular includes the plural andplural encompasses singular, unless specifically stated otherwise. Inaddition, in this application, the use of “or” means “and/or” unlessspecifically stated otherwise, even though “and/or” may be explicitlyused in certain instances. In this application and the appended claims,the articles “a,” “an,” and “the” include plural referents unlessexpressly and unequivocally limited to one referent.

As used herein, “including,” “containing” and like terms are understoodin the context of this application to be synonymous with “comprising”and are therefore open-ended and do not exclude the presence ofadditional undescribed or unrecited elements, materials, phases ormethod steps. As used herein, “consisting of” is understood in thecontext of this application to exclude the presence of any unspecifiedelement, material, phase, or method step. As used herein, “consistingessentially of” is understood in the context of this application toinclude the specified elements, materials, phases, or method steps,where applicable, and to also include any unspecified elements,materials, phases, or method steps that do not materially affect thebasic or novel characteristics of the disclosure.

Whereas particular embodiments of this invention have been describedabove for purposes of illustration, it will be evident to those skilledin the art that numerous variations of the details of the presentinvention may be made without departing from the invention as defined inthe appended claims.

What is claimed is:
 1. A method of making high strength steel sheet witha tensile strength of 800 to 1100 MPa and a hole expansion ratio of atleast 50%, comprising the steps of: reheating a previously cast slab, orretaining the heat from a directly cast slab, above Ar3; hot rolling theslab to final desired thickness; cooling the steel sheet at a rate of50° C. per second to a temperature less than 400° C.; and winding thesteel sheet into a coil.
 2. The method of making high strength steelaccording to claim 1, wherein the temperature Ar3 is a temperaturegreater than the austenite-to-ferrite transformation temperature.
 3. Themethod of making high strength steel according to claim 1, furthercomprising cooling the steel sheet to an acicular ferrite structure. 4.The method of making high strength steel according to claim 1, furthercomprising cooling the steel sheet to a bainitic structure.
 5. Themethod of making high strength steel according to claim 1, furthercomprising the application of a secondary treatment to the steel sheetto promote precipitation reactions for strength preservation or anincrease in strength.
 6. The method of making high strength steelaccording to claim 5, further comprising reheating the steel coil to atemperature below Ac1.
 7. The method of making high strength steelaccording to claim 6, wherein the temperature Ac1 is a temperature above500° C. and below the ferrite-to-austenite phase transformationtemperature.
 8. The method of making high strength steel according toclaim 6, wherein the temperature depends on a time duration anticipatedfor a process employed.
 9. The method of making high strength steelaccording to claim 6, further comprising continuous annealing of thesteel sheet to achieve reduced heating times.
 10. The method of makinghigh strength steel according to claim 9, wherein the reduced durationof the heating time during the continuous annealing allows for the steelsheet to approach a temperature Ac1 temperature while achieving thedesired properties.
 11. A method of making high strength steel sheethaving a tensile strength of approximately 800 MPa and a composition of0.06 weight percent of Carbon, 1.0 weight percent of Mn and 0.1 weightpercent of Si, 0.03 weight percent of Ti and 0.0020 weight percent ofboron, comprising the steps of: reheating a previously cast slab, orretaining the heat from a directly cast slab, above Ar3; than 400° C.;and hot rolling the slab to final desired thickness; cooling the steelsheet at a rate of 50° C. per second to a temperature less winding thesteel sheet into a coil.
 12. The method of making high strength steelaccording to claim 11, further comprising cooling the steel sheet to anacicular ferrite.
 13. The method of making high strength steel accordingto claim 11, further comprising a applying a secondary treatment to thesteel sheet to promote precipitation reactions for strength preservationor an increase in strength.
 14. The method of making high strength steelaccording to claim 13, further comprising reheating the steel coil to atemperature below Act.
 15. The method of making high strength steelaccording to claim 14, wherein the temperature Ac1 is a temperatureabove 500° C. and below the ferrite-to-austenite phase transformationtemperature.
 16. The method of making high strength steel according toclaim 11, wherein the temperature depends on a time duration anticipatedfor a process employed.
 17. The method of making high strength steelaccording to claim 11, further comprising continuous annealing of thesteel sheet to achieve reduced heating times.
 18. The method of makinghigh strength steel according to claim 17, wherein the reduced durationof the heating time during the continuous annealing allows for the steelsheet to approach a temperature Ac1 temperature while achieving thedesired properties.
 19. A method of making high strength steel sheet atensile strength of approximately 1000 MPa and a composition 0.06 weightpercent of C, 1.0 weight percent of Mn, 0.1 weight percent of Si, 0.03weight percent of Ti and 0.0020 weight percent of B, comprising thesteps of: slab, above Ar3; than 400° C.; and reheating a previously castslab, or retaining the heat from a directly cast hot rolling the slab tofinal desired thickness; cooling the steel sheet at a rate of 50° C. persecond to a temperature less winding the steel sheet into a coil.